In a conventional industrial or commercial refrigeration/air conditioning system, gas refrigerant is compressed in a compressor and discharged at a higher temperature and pressure. The compressed gas travels to a condenser, which removes heat from the gas and condenses the gas to liquid. The liquid then travels to an evaporator, which performs its cooling or refrigeration function by evaporating the liquid into gas. As the liquid evaporates, heat is removed from the surrounding environment through the evaporator coils.
One method for feeding liquid refrigerant to the evaporator coils is known as the "flooded" evaporator method. With this method, the evaporator is literally flooded by sending more liquid than the coils can evaporate. Evaporator coils work at optimum efficiency when their entire interior surface remains wet with liquid refrigerant. During the refrigeration process, a portion of the liquid in the evaporator is vaporized into gas. Gas and liquid exit the evaporator and are sent to a gas/liquid separator known as a surge drum. If the system includes more than one evaporator, a single liquid/gas separator known as an accumulator can service several evaporators at the same time.
Liquid from the surge drum or accumulator must be recirculated to the evaporator. Recirculation may be accomplished by mechanical pumps, or by utilizing the gas pressure existing in the system. Gas pressure pumping has several advantages over mechanical pumping, including lower initial installation costs and being virtually maintenance free.
One type of ammonia gas pumping recirculation system is manufactured by H. A. Phillips & Company. In the Phillips system, excess liquid is directed from the accumulator to a dump tank. When the liquid in the dump tank exceeds a predetermined level, high pressure hot gas at approximately 150 psig is fed to the dump tank from downstream of the compressor, thereby forcing liquid from the dump tank into a receiver tank. The liquid in the receiver tank is forced into the evaporators by the gas pressure maintained in the receiver tank.
Although the Phillips gas pumping system has several advantages over mechanical pumping, the Phillips system, as well as other gas pumping systems, can be improved upon. For example, in a single stage (i.e., one compressor) system, it has been found that the gas taken from the output of the compressor is at a much higher pressure than is needed to force liquid from the dump tank. The unused energy associated with the excess pressure is wasted. Because the high pressure gas is also at a very high temperature, heat is discharged into the system, thereby reducing the overall refrigeration efficiency.
U.S. Pat. No. 4,059,968 to Ross (Ross '968 patent) discloses a recirculating refrigeration system that utilizes flash (vaporized) gas at approximately 50 psig (for ammonia) to force liquid refrigerant from a dump tank, 44 or 144. The Ross '968 patent is directed primarily to the effective use of a three-port "economizer" compressor, 30 or 130, having two inlet ports and one outlet port. One inlet port 51 accepts vaporized refrigerant from an accumulator, 41 or 141, and the other inlet port 53 accepts flash gas from a receiver, 16 or 116A. These inputs are compressed and discharged from outlet port 52 at approximately 150 psig.
Although the three-port compressor, 30 or 130, normally has low maintenance costs, it is expensive and can only be used for systems with stable operating conditions. A change in an operating condition, such as evaporating temperature, will cause a change in refrigeration capacity. At refrigeration capacity below 85% of design capacity, the port 53 is exposed to suction pressure, and the "economizer" effect disappears. As a result, two-port compressors are still preferred in recirculating refrigeration systems.
The '968 patent does not solve the problems associated with utilizing pumping gas at a higher pressure than needed to recirculate the liquid. For example, the pressure of the flash gas taken from the flash receiver tank, 16 or 116a, is set by the screw compressor's built-in volumetric ratio. This pressure is limited to a certain range defined by the "intermediate" inlet port 53 and the outlet port 52. Thus, the '968 patent does not adjust the pumping gas pressure to the needs of the system.
Thus, it can be seen that, although gas pumping systems have in general presented improvements over mechanical pumping systems, a system has not been presented that provides the optimum efficiency associated with utilizing pumping gas at the pressure needed.
It is thus an object of this invention to provide a gas pumping recirculation system that is extremely efficient and wastes very little energy.
It is a feature of this invention to provide at least one adjustable pressure regulator valve for controlling the pressure difference between a flash tank and a receiver tank. Pumping gas is taken from the flash tank, and the valve can be set so that the pumping gas pressure is at the level needed to recirculate liquid refrigerant.
It is an advantage of this invention that the compressor is not directly involved in setting the pumping gas pressure, and therefore, the compressor has less requirements placed on it.
It is another advantage of this invention that the pumping gas pressure can be easily adjusted to meet the needs of different systems, or to meet the variable needs within one system.
It is a further advantage that this invention virtually eliminates the waste associated with utilizing pumping gas at a pressure higher than needed to recirculate liquid refrigerant.